The present invention generally relates to the field of seismology and, in particular, to an apparatus and method for processing of seismic reflection data to diminish range dependent tuning effects and enhancing the resolution of true variations of a seismic signal amplitude as a function of range in unstacked common depth point gathers.
As the seismic exploration technology has advanced, the range or separation between seismic sources and seismic receivers has increased. However, at increased ranges due to the convergence of normal moveout curves, the ability to resolve thin subterranean formation tends to be masked by seismic reflection data as a function of range. Specifically, the difference in the length of the seismic signal ray path from the upper and lower surfaces of a thin subterranean formation decreases as the range between a seismic source and a seismic receiver increases. Consequently, a plot of the differences in the arrival times of seismic reflections from these surfaces diminishes as the range increases so as to merge the reflections into a single event. In addition to the merging of seismic events, spurious amplitude variations as a function of range due to the apparent thinning of the seismic event are observed. Such effects are undesirable and detrimental to a correct analysis of range dependent amplitude variations in a seismic signal representing the seismic reflection.
This problem is not improved by conventional processing to correct for normal moveout (NMO) of the seismic signal from the seismic source. In the process of correcting for NMO, stretching of the far range seismic signals occurs. The result is a lowering of the frequency content in the stretched data from the original bandpass; however, resolution is not improved by NMO. The decreasing difference in arrival times with increasing range for thin bed reflections is simply transformed by NMO into a decreasing frequency content with increasing range. As such, true amplitude variations of the seismic signal as a function of range cannot be correctly ascertained due to tuning effects from the differential thinning of seismic events.